Thermal imaging, or thermography, is a recording process wherein images are generated by the use of image-wise modulated thermal energy. There are two commonly known methods for thermal imaging. The first is generally referred to as thermal dye transfer printing, and the second is referred to as direct thermal printing. With thermal dye transfer printing, a desired image is obtained by image-wise heating of a donor element having a dye layer, wherein the application of heat causes at least a portion of the dye to be transferred from the donor element to the imaging media. With direct thermal printing, a desired image is obtained by direct image-wise heating of a thermosensitive recording or imaging media, wherein the application of heat, by chemical or physical processes, changes the color or optical density of the imaging media.
With either method, the image-wise heating is typically accomplished through use of a thermal recording head or printhead. Thermal printheads typically comprise a number of microscopic heating elements, generally resistors, which are usually spaced in a line-wise fashion across the printhead. Typically, a rotatable drum is driven (e.g. by a dc stepper motor, for example) to advance the imaging media past the heating elements of the printhead. When printing a desired image, the thermal printhead prints one line of pixels of the image at a time, with each resistor producing one pixel of the line of pixels on the imaging media. The rotatable drum advances the imaging media as the individual lines of pixels are printed such that the desired image is constructed from a large number of individually printed lines of pixels.
To ensure proper heat transfer from the heating elements to the recording media, the thermal printhead is typically biased toward the rotatable drum, such as with a spring, so that the heating elements firmly contact the imaging media. Generally, the heating elements must be held against the imaging media with at least a certain minimum head pressure in order to achieve a desired image quality. Since the heating elements substantially form a line contact with the imaging media, the head pressure is generally defined as a biasing force of the printhead against the imaging media per unit width of the imaging media.
Some thermal imagers record images on imaging media of various widths. To ensure that the desired contact is attained between the thermal elements and the recording media, conventional thermal imagers often maintain a biasing force on the printhead against the imaging media so that at least the minimum head pressure is achieved for the widest width of imaging media. However, when printing to a narrower width of imaging media, the biasing force necessary to maintain the minimum head pressure on the wider width of media creates a head pressure against the narrower width media that is greater than the minimum head pressure. For example, a thermal imager may record images on 10-inch and 14-inch wide imaging media. If a desired head pressure of 1 kilogram-force per inch (1 kg-f/in.) is desired for a thermal printer recording images on 14-inch wide media, the thermal printhead must be held against the imaging media with a biasing force of 14 kg-f. This same 14 kg-f biasing force creates a head pressure of 1.4 kg-f/in. when a 10-inch wide imaging media is employed.
Although most types of thermal printheads include a protective coating over the heating elements, the increased head pressure against narrower media widths can lead to uneven wearing of the protective coating, with the areas of the printhead corresponding to the narrower media wearing more than those areas corresponding only to wider widths of media. Such uneven wearing can result in uneven heat transfer characteristics across the width of the printhead which, in turn, often translates to uneven densities in a printed image. For example, when printing an image on a wider width of imaging media, the thermal elements from areas of the printhead corresponding to narrower widths of media may produce densities different from those produced by thermal elements from areas of the printhead corresponding to only wider widths of media. The increased head pressure can also result in uneven wearing of the surface of the rotatable drum, which may further increase the likelihood of uneven temperature from the thermal elements to the imaging media.
In light of the above, it is evident that there is a need for improving thermal imaging systems, particularly those used to print images on multiple widths of imaging media, to reduce problems associated with varying head pressure.